Gas separation using molecular sieve SSZ-73

ABSTRACT

The present invention relates to new crystalline, essentially all silicon oxide molecular sieve SSZ-73 prepared using a 3-ethyl-1,3,8,8-tetramethyl-3-azoniabicyclo[3.2.1]octane cation as a structure-directing agent, and its use in gas separations.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to new crystalline, essentially allsilicon oxide molecular sieve SSZ-73, a method for preparing SSZ-73using a 3-ethyl-1,3,8,8-tetramethyl-3-azoniabicyclo[3.2.1]octane cationas a structure directing agent (“SDA”) and uses for SSZ-73.

2. State of the Art

Because of their unique sieving characteristics, as well as theircatalytic properties, crystalline molecular sieves and zeolites areespecially useful in applications such as hydrocarbon conversion, gasdrying and separation. Although many different crystalline molecularsieves have been disclosed, there is a continuing need for new molecularsieves with desirable properties for gas separation and drying,hydrocarbon and chemical conversions, and other applications. Newmolecular sieves may contain novel internal pore architectures,providing enhanced selectivities in these processes.

SUMMARY OF THE INVENTION

The present invention is directed to a family of crystalline molecularsieves with unique properties, referred to herein as “molecular sieveSSZ-73” or simply “SSZ-73”. SSZ-73 is obtained in its silicate form. Theterm “silicate” refers to a molecular sieve containing all silicon oxideor a very high mole ratio of silicon oxide to another oxide.

SSZ-73 is a crystalline molecular sieve comprising essentially allsilicon oxide and having, after calcination, the X-ray diffraction linesof Table II. As used herein, “essentially all silicon oxide” or“essentially all-silica” means that the molecular sieve's crystalstructure is comprised of only silicon oxide or is comprised of siliconoxide and only trace amounts of other oxides, such as aluminum oxide,which may be introduced as impurities in the source of silicon oxide.

In accordance with the present invention there is provided an improvedprocess for separating gasses using a membrane containing a molecularsieve, the improvement comprising using as the molecular sieve acrystalline molecular sieve comprising essentially all silicon oxide andhaving, after calcination, the X-ray diffraction lines of Table II.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a molecular sieve designated herein“molecular sieve SSZ-73” or simply “SSZ-73”. It is believed SSZ-73 has aframework topology similar to that of the molecular sieve designatedSTA-6. That framework topology has been designated “SAS” by the IZA.However, STA-6 is a metallo aluminophosphate, whereas SSZ-73 is asilicon-containing molecular sieve. SSZ-73 is unusual in that it is onlyone-dimensional with small pores, yet has a very large micropore volumedue to its sizeable cages.

SSZ-73 is unusual in that it is only one-dimensional with small pores,yet has a very large micropore volume due to its sizeable cages. SSZ-73has a nitrogen micropore volume of 0.25 cc/gm. This is surprisingly highfor a one-dimensional molecular sieve. SSZ-73 also has an unexpectedlyhigh surface area of about 585 m²/gm.

In preparing SSZ-73, a3-ethyl-1,3,8,8-tetramethyl-3-azoniabicyclo[3.2.1]octane cation is usedas a structure directing agent (“SDA”), also known as a crystallizationtemplate. The SDA useful for making SSZ-73 has the following structure:

The SDA cation is associated with an anion (X⁻) which may be any anionthat is not detrimental to the formation of the SSZ-73. Representativeanions include halogen, e.g., fluoride, chloride, bromide and iodide,hydroxide, acetate, sulfate, tetrafluoroborate, carboxylate, and thelike. Hydroxide is the most preferred anion. The structure directingagent (SDA) may be used to provide hydroxide ion. Thus, it is beneficialto ion exchange, for example, a halide to hydroxide ion.

The 3-ethyl-1,3,8,8-tetramethyl-3-azonia-bicyclo[3.2.1]octane cation SDAcan be prepared by a method similar to that described in U.S. Pat. No.5,268,161, issued Dec. 7, 1993 to Nakagawa, which discloses a method forpreparing 1,3,3,8,8-pentamethyl-3-azoniabicyclo[3.2.1]octane cation.U.S. Pat. No. 5,268,161 is incorporated by reference herein in itsentirety.

In general, SSZ-73 is prepared by contacting an active source of siliconoxide with the 3-ethyl-1,3,8,8-tetramethyl-3-azoniabicyclo[3.2.1]octanecation SDA in the presence of fluoride ion.

SSZ-73 is prepared from a reaction mixture comprising silicon oxide andthe following (expressed in terms of mole ratios):

TABLE A Reaction Mixture Typical Preferred OH—/SiO₂ 0.20–0.80 0.40–0.60Q/SiO₂ 0.20–0.80 0.40–0.60 M_(2/n)/SiO₂   0–0.04    0–0.025 H₂O/SiO₂ 2–10 3–7 HF/SiO₂ 0.20–0.80 0.30–0.60where M is an alkali metal cation, alkaline earth metal cation ormixtures thereof; n is the valence of M (i.e., 1 or 2); Q is a3-ethyl-1,3,8,8-tetramethyl-3-azoniabicyclo[3.2.1]octane cation and F isfluoride.

A preferred active source of silicon oxide is tetraethyl orthosilicate.

If it is desired that SSZ-73 have catalytic activity, small amounts of ametal oxide, such as aluminum oxide, may be introduced into theframework of the SSZ-73. This can be done by adding an active source of,e.g., aluminum oxide into the reaction mixture, resulting insilicoaluminate having a SiO₂/Al₂O₃ mole ratio of about 400/1.

In practice, SSZ-73 is prepared by a process comprising:

-   -   (a) preparing an aqueous solution containing a source(s) of        silicon oxide, a source of fluoride ion and a        3-ethyl-1,3,8,8-tetramethyl-3-azoniabicyclo[3.2.1]octane cation        having an anionic counterion which is not detrimental to the        formation of SSZ-73;    -   (b) maintaining the aqueous solution under conditions sufficient        to form crystals of SSZ-73; and    -   (c) recovering the crystals of SSZ-73.

The reaction mixture is maintained at an elevated temperature until thecrystals of the SSZ-73 are formed. The hydrothermal crystallization isusually conducted under autogenous pressure, at a temperature between100° C. and 200° C., preferably between 135° C. and 180° C. Thecrystallization period is typically greater than 1 day and preferablyfrom about 3 days to about 20 days. The molecular sieve may be preparedusing mild stirring or agitation.

During the hydrothermal crystallization step, the SSZ-73 crystals can beallowed to nucleate spontaneously from the reaction mixture. The use ofSSZ-73 crystals as seed material can be advantageous in decreasing thetime necessary for complete crystallization to occur. In addition,seeding can lead to an increased purity of the product obtained bypromoting the nucleation and/or formation of SSZ-73 over any undesiredphases. When used as seeds, SSZ-73 crystals are added in an amountbetween 0.1 and 10% of the weight of first tetravalent element oxide,e.g. silica, used in the reaction mixture.

Once the molecular sieve crystals have formed, the solid product isseparated from the reaction mixture by standard mechanical separationtechniques such as filtration. The crystals are water-washed and thendried, e.g., at 90° C. to 150° C. for from 8 to 24 hours, to obtain theas-synthesized SSZ-73 crystals. The drying step can be performed atatmospheric pressure or under vacuum.

SSZ-73 as synthesized has the X-ray diffraction lines of Table I below.SSZ-73 has a framework topology comprising essentially all silicon oxideand has a composition, as synthesized (i.e., prior to removal of the SDAfrom the SSZ-73) and in the anhydrous state, comprising silicon oxideand, in terms of mole ratios, the following:

M_(2/n)/SiO₂   0–0.03 Q/SiO₂ 0.02–0.08 F/SiO₂ 0.01–0.10wherein M is an alkali metal cation, alkaline earth metal cation ormixtures thereof; n is the valence of M (i.e., 1 or 2); Q is a3-ethyl-1,3,8,8-tetramethyl-3-azoniabicyclo[3.2.1]octane cation and F isfluoride.

SSZ-73 is characterized by its X-ray diffraction pattern. SSZ-73,as-synthesized, has an X-ray powder diffraction pattern that exhibitsthe characteristic lines shown in Table I.

TABLE I As-Synthesized SSZ-73 Relative Integrated 2 Theta^((a))d-spacing (Angstroms) Intensity (%)^((b)) 8.94 9.88 S 10.67 8.28 W 16.585.34 VS 19.42 4.57 M 20.07 4.42 VS 21.41 4.15 M 25.47 3.49 M 27.69 3.22W–M 30.89 2.89 W 33.51 2.67 W–M ^((a))±0.1 ^((b))The X-ray patternsprovided are based on a relative intensity scale in which the strongestline in the X-ray pattern is assigned a value of 100: W (weak) is lessthan 20; M (medium) is between 20 and 40; S (strong) is between 40 and60; VS (very strong) is greater than 60.

Table IA below shows the X-ray powder diffraction lines foras-synthesized SSZ-73 including actual relative intensities.

TABLE IA As-Synthesized SSZ-73 2 Theta^((a)) d-spacing (Angstroms)Intensity 8.94 9.88 50.2 10.67 8.28 11.1 16.58 5.34 100.0 17.65 5.02 0.519.42 4.57 26.1 20.07 4.42 75.9 20.94 4.24 3.6 21.41 4.15 27.6 24.513.63 10.7 24.96 3.56 6.0 25.47 3.49 31.2 26.57 3.35 7.9 26.75 3.33 6.627.04 3.29 6.2 27.69 3.22 22.8 28.53 3.13 4.5 29.68 3.01 8.2 30.89 2.8913.4 32.62 2.74 4.8 33.19 2.70 9.2 33.51 2.67 20.5 34.91 2.57 7.7 35.622.52 3.0 36.06 2.49 3.1 37.09 2.42 8.5 38.63 2.33 0.9 39.47 2.28 2.140.45 2.23 5.0 40.77 2.21 3.7 ^((a))±0.1

After calcination, the X-ray powder diffraction pattern for SSZ-73exhibits the characteristic lines shown in Table II below.

TABLE II Calcined SSZ-73 Relative Integrated 2 Theta^((a)) d-spacing(Angstroms) Intensity (%) 8.84 10.00 VS 10.69 8.27 W 12.53 7.06 W 16.505.37 W 19.54 4.54 W 19.88 4.46 W 21.49 4.13 W 25.23 3.53 W 27.48 3.24 W33.38 2.68 W ^((a))±0.1

Table IIA below shows the X-ray powder diffraction lines for calcinedSSZ-73 including actual relative intensities.

TABLE IIA Calcined SSZ-73 Relative Integrated 2 Theta^((a)) d-spacing(Angstroms) Intensity (%) 8.84 10.00 100.0 10.69 8.27 9.8 12.53 7.06 5.116.50 5.37 13.1 17.75 4.99 1.1 19.54 4.54 4.9 19.88 4.46 13.1 20.79 4.271.1 21.49 4.13 4.4 24.35 3.65 1.5 24.96 3.56 0.7 25.23 3.53 7.9 26.543.36 1.0 26.79 3.33 1.8 26.99 3.30 0.6 27.48 3.24 4.2 28.27 3.15 1.430.81 2.90 2.3 32.32 2.77 0.7 32.34 2.77 0.4 32.91 2.72 2.5 33.38 2.683.6 34.96 2.56 0.1 35.21 2.55 0.5 35.36 2.54 0.3 ^((a))±0.1

The X-ray powder diffraction patterns were determined by standardtechniques. The radiation was the K-alpha/doublet of copper. The peakheights and the positions, as a function of 2θ where θ is the Braggangle, were read from the relative intensities of the peaks, and d, theinterplanar spacing in Angstroms corresponding to the recorded lines,can be calculated.

The variation in the scattering angle (two theta) measurements, due toinstrument error and to differences between individual samples, isestimated at ±0.1 degrees.

Representative peaks from the X-ray diffraction pattern of calcinedSSZ-73 are shown in Table II. Calcination can result in changes in theintensities of the peaks as compared to patterns of the “as-synthesized”material, as well as minor shifts in the diffraction pattern.

Crystalline SSZ-73 can be used as-synthesized, but preferably will bethermally treated (calcined). Usually, it is desirable to remove thealkali metal cation (if any) by ion exchange and replace it withhydrogen, ammonium, or any desired metal ion.

SSZ-73 can be formed into a wide variety of physical shapes. Generallyspeaking, the molecular sieve can be in the form of a powder, a granule,or a molded product, such as extrudate having a particle size sufficientto pass through a 2-mesh (Tyler) screen and be retained on a 400-mesh(Tyler) screen. In cases where the catalyst is molded, such as byextrusion with an organic binder, the SSZ-73 can be extruded beforedrying, or, dried or partially dried and then extruded.

SSZ-73 can be composited with other materials resistant to thetemperatures and other conditions employed in organic conversionprocesses. Such matrix materials include active and inactive materialsand synthetic or naturally occurring zeolites as well as inorganicmaterials such as clays, silica and metal oxides. Examples of suchmaterials and the manner in which they can be used are disclosed in U.S.Pat. No. 4,910,006, issued May 20, 1990 to Zones et al., and U.S. Pat.No. 5,316,753, issued May 31, 1994 to Nakagawa, both of which areincorporated by reference herein in their entirety.

The molecular sieve of the present invention can be used to separategasses. For example, it can be used to separate carbon dioxide fromnatural gas. Typically, the molecular sieve is used as a component in amembrane that is used to separate the gasses. Examples of such membranesare disclosed in U.S. Pat. No. 6,508,860, issued Jan. 21, 2003 toKulkarni et al., which is incorporated by reference herein in itsentirety.

EXAMPLES

The following examples demonstrate but do not limit the presentinvention.

Example 1 Synthesis of SSZ-73

To a Teflon cup for a 23 ml Parr stainless steel reactor, 5 millimolesof 3-ethyl-1,3,8,8-tetramethyl-3-azoniabicyclo[3.2.1]octane hydroxideSDA and 2.10 grams of tetraethyl orthosilicate were added. The aqueoussolution of the SDA in its hydroxide form will hydrolyze theorthosilicate ester. The mix of the two reactants was left in a hoodwithout a top to allow the ethanol and water (for the most part) toevaporate over 5–7 days until the internal contents appeared to be dry.The reactor (which had been tared) was re-weighed and a small amount ofwater was added back in to adjust the H₂O/SiO₂ mole ratio to 3.5. Then,0.20 grams of 48–52% HF was added drop wise and the contents were mixedwith a plastic spatula. A thick gel set up. The reactor was closed andheated for 9 days at 150° C. and 43 RPM. The reactor was removed fromthe oven, cooled to room temperature and a sample was taken for ScanningElectron Microscopy. No crystals were seen, so the reaction was run inseries of 6 day increments until a product was seen at 27 days. Thecontents of the reactor were then collected in a fritted filter withcopious water washing. After drying, the crystalline product was found,by x-ray diffraction, to be SSZ-73. Subsequent runs can have theirreaction time about halved by adding seed material.

Example 2 Calcination of SSZ-73

The material from Example 1 was calcined in the following manner. A thinbed of material was heated in a muffle furnace from room temperature to120° C. at a rate of 1° C. per minute and held at 120° C. for threehours. The temperature is then ramped up to 540° C. at the same rate andheld at this temperature for five hours, after which it was increased to594° C. and held there for another five hours. A 50/50 mixture of airand nitrogen was passed over the SSZ-73 at a rate of 20 standard cubicfeet (0.57 standard cubic meters) per minute during heating.

1. In a process for separating gasses using a membrane containing amolecular sieve, the improvement comprising using as the molecular sievea crystalline molecular sieve comprising essentially all silicon oxideand having, after calcination, the X-ray diffraction lines of Table II.